The present invention relates to an apparatus for automatically focusing objects to be viewed in optical instruments with the aid of an illuminating beam of light, wherein a measuring point of light is produced on the surface of the object by a separate, eccentrically transmitted measuring beam. The measuring point is reproduced by the reflected measuring beam, which is conducted eccentrically along a path formed by blocking out a portion of the originally transmitted measuring beams, on a photoelectric device. Upon migration of the object plane from the focal plane of the objective, the signals of the photoelectric device actuate a control device, which returns the object plane to the focal plane. An apparatus of this type is known from No. DE-PS 21 02 922.
An apparatus is described in No. AT-P 353 497 for the automatic focusing of instruments on different object planes, in which, between the objective and the photoelectric sensors, an optical grating with beam splitting properties is inserted. This grating is displaceable in a controlled manner in the direction of the optical axis of the objective and may further be oscillated perpendicularly to the axis. Alternatively, the optical grating may be replaced by double wedge which is variable in thickness but otherwise stationary.
While the apparatus of No. AT-P 353 497 has the disadvantage that the system is a passive one in which the evaluation of the sharpness of the image is effected solely from the image "offered" in the corresponding intermediate image plane, the apparatus disclosed in No. DE-PS 21 02 922 has the disadvantage that in the case of an intentional setting of another object plane as the focal plane, the diameter of the spot on the object is enlarged and the spot on the object surface wanders out of the center point. Thus, it is possible in this instance that, as a result of the object surface topography, the migrating spot would impinge on a detail of the object positioned differently in the z-axis (the z-axis extends in the direction of the optical axis of the objective of the optical instrument). It is not always possible to obtain a defined offset setting in the case of objects structured in this manner with the aid of the known apparatus. A further disadvantage consists of the fact that, as the result of the spot enlargement during defocusing, a displacement of the center of intensity on bright/dark structural edges of the object occurs; this leads to an impairment of a defined offset setting, as the offset depends on the position of the center of intensity of the spot.